This invention relates to a method of fabricating microelectronic circuit elements and particularly to a method of forming electrically isolated regions of polycrystalline silicon on an insulating substrate.
Polycrystalline silicon, more commonly referred to as polysilicon, is used extensively in silicon integrated circuit technology as a first layer of isolated conductive elements, particularly in structures where a second conductive layer is to be formed. It is also advantageously used as a conductor in MOS structures and techniques known as "silicon gate."
In the usual processing of the foregoing structures, the polysilicon is deposited over the entire surface of a wafer and then it is selectively etched through a mask. The mask is usually a material such as silicon nitride or alumina which itself is formed by photolithographic masking and etching techniques into a pattern which is duplicated by the polysilicon. The etching step that forms the polysilicon pattern is referred to as silicon etch and employs an etchant that removes the polysilicon from the unwanted regions without attacking the silicon nitride or alumina, and thereby leaves polysilicon directly beneath the silicon nitride or alumina. Finally, the silicon nitride or alumina is removed by etching.
The foregoing silicon etch process has several disadvantages. One drawback is that the polysilicon is undesirably undercut beneath the silicon nitride or alumina. When a layer of material suffers undercutting, it is difficult to deposit and pattern thereover subsequent layers which do not have discontinuities at the undercut boundaries. Another drawback is that the polysilicon structure that remains after the silicon nitride or alumina is removed is higher or thicker than the etched region surrounding it. Such a structure does not readily faciliate the formation of selective ion implants beneath the polysilicon.